1. Field of the Invention
The present invention relates to a variable magnification optical system and an imaging apparatus that are used in a video camera, an electronic still camera and the like. In particular, the present invention relates to a variable magnification optical system that is appropriate for use in a monitor camera, and which is usable for both of a visible light range and a near-infrared light range. Further, the present invention relates to an imaging apparatus including the variable magnification optical system.
2. Description of the Related Art
Conventionally, monitor cameras are used to prevent crimes, to record images, or the like. Optical systems for the monitor cameras need to be small and at low cost. The optical systems need to have large aperture ratios so that subjects are identified even in low-illumination photography conditions, as well as having high optical performance. Further, a demand for monitor cameras with variable magnification functions increased in recent years. Naturally, the main trend of optical systems for monitor cameras will be variable magnification optical systems. The variable magnification optical system that can be used in monitor cameras is disclosed, for example, in U.S. Pat. No. 7,167,318 (Patent Document 1). The variable magnification optical system disclosed in Patent Document 1 is a two-group zoom optical system composed of a first lens group having a negative refractive power and a second lens group having a positive refractive power. In the variable magnification optical system disclosed in Patent Document 1, each of the first lens group and the second lens group is composed of three lenses. Therefore, the variable magnification optical system disclosed in Patent Document 1 is compact.
Meanwhile, a monitor camera performs photography by visible light during day time, and performs photography by near-infrared light during night time or after dark in many cases. Therefore, the monitor camera needs to cope with both of a visible light range and a near-infrared light range. Therefore, a lens system of the monitor camera needs to be structured in such a manner that chromatic aberrations are corrected in an excellent manner for the range including visible light through near-infrared light. As a variable magnification optical system that can be used for both of the visible light range and the near-infrared light range, a variable magnification optical system disclosed in U.S. Pat. No. 7,050,240 (Patent Document 2) is known. The variable magnification optical system disclosed in Patent Document 2 has two-group zoom structure, and each of the first lens group and the second lens group of the variable magnification optical system is composed of four lenses. Further, aberrations are corrected in an excellent manner for both of the visible light range and the near-infrared light range.
Meanwhile, imaging devices, such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor), are mounted on cameras in the aforementioned fields. In recent years, the resolution of the imaging devices became higher, and a demand for high-quality images or video images by use of monitor cameras increased. Especially, a demand for high-performance variable magnification optical systems that can cope with imaging devices having pixel numbers of 1 million or higher is increasing. However, in conventional optical systems, it was difficult to achieve high-performance optical systems that can cope with the increase in resolution in recent years, while the large aperture ratio and the compactness (small size or the like), which are necessary for use in monitor cameras, are maintained.